Consider this: There was a time, maybe a half-century ago, when railroads finagled a way around overloaded trains or engines facing mountainous terrain. If the company plied a train with too much weight, workers hitched it to an engine up front and then hitched another engine behind the train to give it added momentum up steep hills.

Your metabolism works something like a train engine. In midlife, metabolism slows like an overloaded train facing a heck of a hill. Although the metabolic train slows, it doesn't have to grind to a halt. All it needs is a little extra push, with the help of metabolism boosting foods and a bit of exercise.

A Crash Course in MetabolismDon't curse your metabolism. You need it for every breath you take.

All forms of life—from simple algae in a backyard swimming pool to your dog or cat to the human body—depend on hundreds of carefully regulated, and simultaneous, chemical reactions to stay alive.When your body digests food, it breaks down the carbohydrates, fats, and proteins you eat into smaller compounds that are absorbed by the blood.

Carbohydrates (found primarily in fruits, vegetables, grains, potatoes, and candy, for example) break down into glucose.

Fats (from either vegetable oils like olive oil or animal fats like butter) break down into fatty acids and glycerol.

Carbohydrates, protein, and fat contain calories. Fiber is a form of carbohydrate that produces no calories and aids in weight control by providing bulk, which makes food more filling.Your basal metabolic rate (BMR) is the number of calories you expend to sustain life—the energy spent to keep the heart beating, the lungs expanding, or the body humming along at a healthy 98.6 degrees F. But you also expend energy when you move—walk, run, cycle, garden, dance, chase a bus, or work out at the gym.

Both the conversion of food to energy and your body's use of that energy—your metabolism—play a key role in weight control. If you take in more energy than your body needs, you gain weight. It doesn't matter if all those extra calories came in the form of a double cheeseburger, two extra slices of garlic bread with spaghetti, a second pat of butter slapped on your mashed potatoes, or a glass or two of wine with dinner every night. Extra calories in all forms—protein, fat, carbohydrates, and alcohol—add up to weight gain.

If your body has an excess of calories from any source, it rearranges them into stores of fat and carbohydrates, to be drawn upon between meals and overnight in between fuel deliveries. If you take in more energy than you burn, you gain weight as body fat.

[header = The Natural Slowdown]

The Natural SlowdownAlmost imperceptibly at first, a woman's metabolic train slows as the birthdays add up.

Somewhere in her thirties, a woman's metabolism approaches a big hill and starts to slow about 5% every decade. That means that if a moderately active 35-year-old woman ate a set number of calories a day to maintain a weight of 140 pounds, she might gain weight eating the same number of calories at age 45.

For many women at midlife, the weight gain is so gradual they don't notice it until they try to slip into their "skinny" jeans and can't close the zipper. Or they find they need to notch their belt at the next larger hole.

"If you gain 1 pound a year, after a year it won't seem like much, but after 10 years, it adds up," says Geoffrey Redmond, MD, endocrinologist and the author of The Good News about Women's Hormones.

The only way to burn calories over and above your basal metabolic rate is to expend more effort. Regular, intentional, calorie-burning exercise goes a long way to counteracting the falloff in metabolism that occurs at midlife. (Our exercises show you how.)

Scientists don't yet know why, but African-American women have lower basal metabolic rates than Caucasian women, according to a study from the National Institutes of Health. "It may turn out that African-American women may need to exercise longer, more frequently, and with greater intensity than others to keep the weight off," says Jana Klauer, MD, a research fellow at the New York Obesity Research Center at St. Luke's-Roosevelt Hospital and Columbia University.

When it comes to metabolism, the sexes aren't equal, either. One study at Jean Meyer USDA Human Nutrition Research Center on Aging at Tufts University in Boston found that women's "resting energy expenditure"—the number of calories expended on day-to-day activities—declined significantly as they aged, but the decline wasn't nearly so noticeable in aging men.

Hormones, too, influence metabolism in various ways. Cortisone and hydrocortisone are crucial to regulating the metabolism of proteins, carbohydrates, and fats. Insulin from the beta cells of the pancreas triggers cells to take in glucose and amino acids. And a hormone called thyroxine responds to another hormone, called Thyroid Stimulating Hormone (TSH), to prompt many cells to increase their metabolic rate, their growth, even their heat production.

[header = Muscle Matters]

Muscle MattersAs if they don't have it easy enough sometimes, men hold a metabolic edge over women because they have more muscle, and muscles are the "workhorses" of the body, says Ann Grandjean, EdD, executive director of the International Center for Sports Nutrition in Omaha, Nebraska.

Consider the case of Janet, a former college athlete who felt "fat" even though she hadn't gained weight. Like many women, she had gained fat and lost muscle over the years. Janet weighed 140 pounds at age 35. At that time, 23% of her body consisted of fat. (Experts consider 23 to 33% body fat healthy for women ages 40 to 59). Back then, 32 of Janet's 140 pounds were fat. The rest—108 pounds—consisted of bone, muscle, water, and internal organs.

By the time Janet reached age 51, her body fat had increased from 23 percent to 30 percent, yet her weight had remained the same. Her body now contains 42 pounds of fat, 10 more pounds of fat than at age 35. At the same time, Janet had lost approximately 5 pounds of muscle.When women gain fat and lose muscle, two things happen:

Fat isn't as dense as muscle, so any fat gained takes up more space than muscle. Even if you haven't gained weight on the scale, your body can appear larger, and your clothing size may even increase.

Because muscle burns more calories than fat, your metabolism slows and you burn fewer calories, which can contribute to weight gain if you don't make adjustments in your calorie consumption.

Janet worries that if she isn't extraordinarily careful about what she eats, she will gain weight—and with good reason. The amount of lean body mass you have is an important factor in determining the rate at which you burn calories. If lean body mass drops, metabolism drops.

Every pound of muscle a woman loses slashes the number of calories she burns by as many as 30 calories a day. If she loses 10 pounds of muscle over 3 decades, she could burn 300 fewer calories each day, or a whopping 2,100 fewer calories each week. By the time she celebrates her 55th birthday, she could have lost as many as 15 pounds of muscle, and now burn 450 fewer calories each and every day.

What this means for Janet—and other women in their forties and fifties—is that maintaining muscle mass is critical as the birthdays add up. Lean muscle matters because there's so much of it. Calorie-burning muscle accounts for approximately 40% of the body mass of a normal-weight woman—that's 56 pounds for a 140-pound woman like Janet—so it's a major factor in energy.

Here's some good news: Because muscle mass is linked directly to metabolic rate, women can give their metabolic engines a boost with weight training and other forms of exercise that builds muscle.

Eating Too Little?As a young woman, 58-year-old Colleen lost weight easily when she crash dieted, shedding pounds so quickly that she began to feel invincible. Sometimes it's easier for busy women to skip exercise, opting for yet another 800-calorie-a-day crash diet instead. Yet the older you get, the less productive crash dieting is. Dieting alone results in a loss of some lean weight—that is, muscle—in addition to fat. But if the weight is regained by returning to a normal diet, it will be in the form of fat.

To supply the red blood cells, brain, and nerves with glucose after the glycogen is gone, the body opts for biology's version of plan B: It breaks down muscle and liver tissue to make the glucose it needs.

The metabolic catch-22? As the lean, protein-rich liver breaks down and shrinks, it works less in order to "spend" less energy. The same is true of muscles: As they waste away, they also work less and demand less fuel. Metabolism, in effect, grinds to a crawl.

As metabolism slows, fat loss also slows, so much that dieters on crash diets or very low calorie plans lose less fat than they would on a sensible reduced-calorie program. In most cases, this means that crash dieters who see big losses on the scale may actually lose less fat than dieters who opt to eat—and enjoy—more food.

Low-carbohydrate diets also trigger the body to draw upon its glycogen stores to provide glucose to the cells. Once those glycogen reserves are gone, the body turns to protein, its only remaining source of glucose. Although low-carbohydrate diets are high in protein—focusing on lots of fish, meats, and cheese—the body still breaks down lean tissue, like muscle, to meet its energy needs. Luckily, consuming a satisfying 1,500 calories a day will help most women avoid the slowdown in metabolism that occurs with low calorie intake.

Morning, Noon, or Night: Just Do ItAside from burning calories, exercise also boosts your metabolism—and depending on when you do it, it could have even more potent calorie-burning power.

In one study, done at the University of Chicago, scientists discovered that male athletes who exercised at 6 o'clock in the evening or late at night, around 1 o'clock, showed significant increases in levels of cortisol and thyrotropin—two hormones that are critical in energy metabolism.

Those who exercised in the morning or afternoon had much smaller increases in the same hormones and smaller drops in blood glucose. (Comparable studies still need to be done on women.)

"Although the effects of exercising late at night were the most extreme, we also saw significant increases of cortisol and thyrotropin in evening exercisers," said Orfeu Buxton, PhD, a postdoctoral fellow in endocrinology at the University of Chicago and lead author of the study. "Right now, we're not sure what that means. But we do know the differences in the hormone levels are very large depending on whether you exercise at night or evening, or early in the day."

While Buxton said it's premature to recommend exercising at one time of day over another, he will say his study broke ground by looking at the hormonal effects of exercising at night. "Right now, what's important is getting exercise in at some part of the day," says Buxton. "It will be years before we know if evening is definitely better." Until we know more, most experts agree that the best time to exercise is when you have the time and the energy and are most likely to actually do it.

[header = Magic Pill Hype]

"Magic Pill" HypeAn old marketing axiom dictates, "Find a need and fill it." Enter supplements touted for their alleged ability to boost metabolism and burn calories, aiding in weight loss—such as ephedra (also known as ma huang), tiratricol, and others.

These miracle cures aren't so miraculous—or safe—after all.

"Most of the studies show they have virtually no effect, or the effects are insignificant," says John Acquaviva, PhD, an exercise physiologist and assistant professor of physical education at Roanoke College in Salem, Virginia.

For example, you may only burn an extra 25 or 30 calories a day. When you burn a total of 2,000 calories a day, that's nothing. "If you're burning 30 more calories a day through taking these products, it's not worth the risk," says Acquaviva.

One study—conducted by the Department of Medicine at the University of California, San Francisco, and the California Poison Control System in San Francisco—found that dietary supplements containing ephedra alkaloids (ma huang) cause high blood pressure, heart palpitations, and central nervous system abnormalities in some people. Some deaths have also been linked to this controversial supplement.

Almost half of the 140 cases the scientists examined for the study were considered "definitely or probably related or possibly related" to the use of the dietary supplements. The most common problem was high blood pressure, but researchers also documented strokes and seizures.

"For women who have underlying medical conditions they didn't know about, this stuff can be very dangerous," says Miriam Nelson, PhD, director of the Center for Physical Fitness, School of Nutrition Science Policy, at Tufts University in Boston.

The FDA banned the sale of ephedra in February 2004. They warn that "metabolism-boosting" products containing tiratricol, also known as triiodothyroacetic acid (TRIAC), marketed through health food stores, gymnasiums, and fitness centers, is a potent thyroid hormone that can trigger heart attacks and strokes. Other side effects include insomnia, sweating, nervousness, and diarrhea.

Just because a product claims to be "herbal" or "natural" doesn't mean it's safe or effective. At best, products so labeled could be a waste of money. At worst, serious damage to one or more vital organs could occur, especially when used with other drugs. (Check out these 7 "All-Natural" Foods That Aren't.)

Since over-the-counter supplements and herbal remedies are not regulated by the FDA, they can put "miraculous" language on their labels to entice you.